Aircraft engines burn fuel at very high temperatures. In combustion areas that operate at these high temperatures special metallic liners are required to control combustion temperatures and prevent thermal damage to engine casings and components. This is accomplished through controlled mixing of combustion exhaust and relatively cool gas.
Combustion primarily takes place in two areas in an aircraft engine, the combustion chamber and the augmentor. In the combustion chamber, compressed air from the compressor is heated prior to its expansion that rotates the turbine. In the augmentor, commonly known as the afterburner, fuel is mixed with the turbine exhaust and ignited in order to raise the pressure and velocity of the gases exiting the engine nozzle.
The afterburner chamber forms a very large combustion area which requires a large cylindrical combustion liner. The typical afterburner liner has considerable weight since it must be strong enough to support itself in flight conditions and be capable of withstanding high temperatures generated during afterburner useage. In order to cope with this environment afterburner liners are often coated with a ceramic material to reduce metal erosion and increase liner life expectancy. Some erosion and other thermal damage is, however, common and afterburner liners often need periodic replacement or refurbishment.
Cracking and other damage to liners often stems from low cycle thermal fatigue. Hot areas of liner material are often surrounded by cooler areas and as a result undergo repeated annealing and restraining which can result in cracking. Further the trailing edge of cooling air holes often oxidize and initiate burn throughs which are propagated by oxidation at adjacent cooling holes.
During conventional repairs, holes are drilled at the end of cracks to prevent further cracking. This is an imperfect procedure since often the end of a crack is very difficult to determine without X-ray or other inspection means not available in the field.
When a liner is damaged by a burn through the liner must be removed from the engine, the ceramic coating stripped and the burned through area repaired by welding. After welding the liner needs to be heat treated, recoated with the ceramic and reinstalled in the engine. This process of removal and repair can take days or weeks and substantially increases the amount of time the engine is out of commission. Further, liners repaired by this conventional method rarely have more than 50% of the life expectancy of a new part. Thus conventional repairs are costly in both time and manpower without fully accomplishing their intended purpose of renewing liners.
Both combustion chamber liners and afterburner combustion liners often have specific hot spots that reoccur on particular engines. Although random hot spots do occur it is often possible to determine a combustion flame profile and locate regions of a particular model liner most susceptible to heat damage. Combustion liner design therefore takes into consideration where the liners are heated the most, i.e., hot spots. Since liners are generally cylindrical components, the amount of material required to withstand a hot spot frequently determines the thickness of the entire liner at a particular axial location. Thus liner weight is substantially increased by the need to withstand hot spots. Other methods of taking into consideration combustion hot spots require a mix of materials or designs of varying liner circumferential thicknesses; use of these methods present difficult problems in manufacturing and increase liner manufacturing cost.
In view of the above it is an object of this invention to provide a method and apparatus for repair of combustion liners which substantially reduces the amount of time and effort required to repair damage typical to such liners.
It is a further object of this invention to provide a means of improving combustion liners by providing for combustion hot spots without adding substantial weight or manufacturing cost to such liners.